Effect of Pressure on Boiling or Condensation

The saturation temperature (the temperature where boiling or condensation occurs) of a liquid or vapor increases or decreases according to the pressure exerted on it.

In the air conditioning system, high-pressure liquid Refrigerant-134a from the A/C condenser core (19712) enters the fixed orifice tube in the liquid line. When the liquid Refrigerant-134a is released into the A/C evaporator core (19860) by the orifice tube, the resulting decrease in pressure and partial boiling lowers its temperature to its new boiling point. As the Refrigerant-134a flows through the A/C evaporator core , passenger compartment air passes over the outside surface of the core. As Refrigerant-134a boils, it absorbs heat from the air and cools the passenger compartment. The heat from the passenger compartment is absorbed by the boiling refrigerant and hidden in the vapor. The refrigeration cycle is now under way. To complete cycle, the following remains to be done:

1. Dispose of the heat in the vapor.

2. Convert the vapor back to liquid for reuse.

3. Return the liquid to the starting point in the refrigeration cycle.

   The A/C compressor (19703) and A/C condenser core perform these functions. The compressor pulls the refrigerant vapor (containing the hidden heat) out of the A/C evaporator core and suction accumulator/drier (19C836) and forces it under high-pressure into the A/C condenser core which is located in the outside air stream at the front of the vehicle. The increased pressure in the A/C condenser core raises the Refrigerant-134a condensation or saturation temperature to a point higher than that of the outside air. As the heat transfers from the hot vapor to the cooler air, the Refrigerant-134a condenses back to a high-pressure liquid. The liquid under high-pressure now returns through the condenser to evaporator tube (19835) to the A/C evaporator core orifice (19D990) for reuse.

   It may seem difficult to understand how heat can be transferred from a comparatively cooler vehicle passenger compartment to the hot outside air. The answer lies in the difference between the refrigerant pressure that exists in the A/C evaporator core and the pressure that exists in the A/C condenser core . In the A/C evaporator core , the A/C compressor suction reduces the pressure and the boiling point below the temperature of the passenger compartment. Heat transfers from the passenger compartment to the boiling refrigerant. In the A/C condenser core , the A/C compressor raises the condensation point above the temperature of the outside air. The heat transfers from the condensing refrigerant to the outside air. The A/C evaporator core orifice and the A/C compressor simply create pressure conditions that permit the laws of nature to function.